The production of organic chemicals by microorganisms is well known to those familiar with the fermentation art. These chemicals are produced by the microorganisms in dilute aqueous solutions, generally from about 1% to 10% by weight, so that their recovery in pure form involves separation from a large quantity of water. The expense of such separation has been so great that production of these chemicals by fermentation has not been able to compete with their production based on petroleum fossil fuel sources. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interests in such fermentation reactions which can convert carbohydrates that are renewable raw materials into simple organic chemicals.
For these reasons, it is desirable to develop a low-cost process for seperation of organic chemicals from dilute aqueous solutions. Various methods have been proposed for such separations. These include solvent extraction, freeze crystallization, distillation and selective adsorption.
It is well known that activated carbon selectively adsorbs organic chemicals from aqueous solutions. The adsorbed chemicals can then be desorbed by various techniques such as heating, displacing the adsorbed material with steam or desorbing the material with a solvent. Spence, U.S. Pat. No. 2,422,504, disclosed a process for recovering lower fatty acids from a dilute aqueous solution. This involved first adsorbing the material on activated carbon. Then the carbon was heated to 100.degree. C. under vacuum to remove a part of the water plus some acid. Finally, the acid and remaining water were extracted with a solvent and the solvent and acids were separated. The expensive heating step detracts from this process.
Baieri, Canadian Pat. No. 978,308, disclosed a method for recovering acetic acid and furfural from sulfite waste liquors obtained as a by-product of paper manufacture. In his process, the liquor was passed through a stream stripper and then through a carbon column to adsorb some of the organic chemicals present. The chemicals were subsequently desorbed using a solvent which was at least partially vaporized. Suggested solvents were the lower alcohols, acetone, benzene, and ether. However, the examples were directed to the recovery of acetic acid with an alcohol which gave an ester or mixtures of ester, alcohol and acid. The mixed product made this process unattractive.
Baieri, U.S. Pat. No. 4,016,180, disclosed a two-stage adsorption-desorption process for concentrating sulfite waste liquors. The liquors are passed through a carbon column to adsorb the organic chemicals. A more concentrated solution of the chemicals was then obtained by the use of either steam or a solvent to desorb the material. Examples include the use of steam, methanol or ethanol to remove acetic acid from the carbon. When alcohols were used as desorbents, mixtures containing esters were obtained. The process described by Baieri requires quite complex equipment with both upflow and downflow of the eluting solvent.
We have now discovered a greatly simplified adsorption process for the concentration of organic compounds from dilute aqueous solutions which is energy-efficient and which permits the use of simple equipment. This process is particularly suitable for the concentration of organic compounds obtained by microbial fermentations.